In radiofrequency transmission equipment, the output signals generated by the frequency generator are used to select a channel in the case of a transmission or to assign the equipment to a channel to be used in the case of reception. For example, an output frequency Fs is chosen from a set of predefined discrete frequencies expressed in the form Fs=k×ΔFs, with kmin≦k≦kmax, k an integer and ΔFs a frequency chosen as a function of the needs of the application.
In a basic architecture, a frequency generator includes a local oscillator for generating a reference signal. This reference signal, which has a reference frequency Fref, is applied to a phase-locked loop in which the reference signal is divided by a first divider. The signal obtained that has a comparison frequency Fcomp is applied to a first input of the phase comparator. The output signal from the frequency generator is also subject to a frequency division in a second frequency divider and is then applied to a second input of the phase comparator. The comparator provides the result of the comparison between the first input and the second input via a low-pass filter to a voltage-controlled oscillator that generates the output signal with the frequency Fs as a function of its input signal. Generally, such frequency generators are controlled by a controller that chooses the division ratios of the first and second divider as a function of the desired output frequency Fs.
Part of the output signal from the voltage-controlled oscillator is amplified for use by a radiofrequency transmission and/or reception chain and another part is returned, as previously described, to the comparator through the second divider.
Such an architecture makes it possible to produce very fine frequency steps ΔFs relative to the output frequency Fs. Cases exist in which the spectrum delivered by the frequency generator is influenced by parasitic lines, which in particular appear in the case where Fs is close to a harmonic of the reference frequency Fref of the reference signal directly applied to the phase-locked loop or is close to a harmonic of the comparison frequency Fcomp.
In order to offset this drawback, the architecture previously described is often modified by adding an additional circuit between the local oscillator and the phase-locked loop, for example a basic circuit of a direct digital synthesizer (DDS) or a second phase-locked loop. This additional circuit is also controlled by the controller so as to eliminate cases of unfavorable relationships between the reference frequency Fref applied to the phase-locked loop, the comparison frequency Fcomp and the output frequency Fs.
Nevertheless, such a solution requires a complex additional circuit that causes significant additional consumption and an unacceptable space requirement for the use of such a frequency generator in portable equipment. Furthermore, the additional circuit for generating a variable reference frequency causes a deterioration of the quality of the signal generated by the additional circuit compared to that of the original reference signal.